Quantitative and qualitative approaches to assess tree vigor and stand health in dry pine forests

Abstract

Despite a critical need to evaluate effectiveness of forest treatments in improving stand health, practitioners lack quantitative, repeatable metrics to assess tree vigor and stand health. We evaluated canopy and whole tree attributes of ponderosa pine (Pinus ponderosa Dougl. Ex Laws) related to carbon balance, water balance, and susceptibility to insects and pathogens in dry, pine-dominated forest stands during a multi-year drought, an environmental challenge to stand resilience. Metrics of trees in two unmanaged, and seven treated forested stands, in both uplands and lowlands to develop the quantitative approach. Whole tree and crown attributes including needle length and color, branchlet length and diameter, needle retention (needle ages and retention within ages), and frequency of insects, fungi, and abiotic needle damage were statistically selected to assess tree vigor. Cluster analysis of vigor attributes revealed that trees responded or persisted independently within a forest treatment; forest treatments did not necessarily yield similar tree responses within a stand. A rapid, qualitative assessment was developed to rank trees as low, average, and above-average vigor. To demonstrate an application of our approach, trees were ranked annually over six years in most stands, as well as in a stand where the prescription was adjusted due to the evaluation. The proportion of trees in the three tree vigor ranks differed, suggesting differing levels of stand health. Quantitative metrics and qualitative ranking of tree vigor could assist in selecting trees to be retained to meet specific management objectives, to evaluate treatment implementation, and to monitor post-treatment changes in stand health.

Assessment of Ponderosa Pine Vigor Using 4-Band Aerial Imagery in South Central Oregon: Crown Objects to Landscapes

“…Our goals in this study were to (1) determine whether trees morphologically identified as low, average and above-average vigor [32] could be spectrally identified; (2) develop an operational workflow for classifying ITCs across a dry pine forest landscape using high-resolution 4 band imagery; and (3) apply the classification to remotely assess the proportions of low vs. 'not low' vigor classified ITCs in several silvicultural and fuel treatments, in two topographic positions. This approach is intended to be a proof of concept model for linking ground-level observed tree vigor with remotely detected tree vigor and applying this model at the landscape scale to assess relative effectiveness of various treatments in improving stand health.…”

“…Annual precipitation totals averaged 15% below the 1998 -2018 mean for 5 years (2001,2002,2014,2015,2018), and 15% above average for 3 years (2006, 2011, 2016) [33]. Although precipitation was greater in 2016, most of that precipitation fell in May so that by August severe physiological tree drought stress had also developed [32]. Overall, the PP in the study area showed visible signs of severe drought stress -lower needle elongation, low leaf cell turgor at noon, low retention of older needles, and chlorosis by August [32].U.S.…”

“…Although precipitation was greater in 2016, most of that precipitation fell in May so that by August severe physiological tree drought stress had also developed [32]. Overall, the PP in the study area showed visible signs of severe drought stress -lower needle elongation, low leaf cell turgor at noon, low retention of older needles, and chlorosis by August [32].U.S. Drought Monitor archived data classified the study area into the moderate or greater drought intensity class for all years 2001-2016 except for 2006, 2008, 2009, and 2011 [34].…”

“…Drought Monitor archived data classified the study area into the moderate or greater drought intensity class for all years 2001-2016 except for 2006, 2008, 2009, and 2011 [34]. Overall, PP at this site showed visible signs of severe drought stress -low leaf cell turgor at noon, lower needle elongation, low retention of older needles, and chlorosis suggesting drought-induced oxidative stress [32].…”

“…The selection of tree vigor attributes is based on previous work that established the relevance of the attributes in describing tree vigor [39][40][41]. The vigor attribute measurements are summarized in the appendix [32,42]. Tree foliage was sampled by clipping 3-5 secondary branchlets proximal to the terminus in mid-crown with a 13 m pole pruner.…”